Section III Lab notes Flashcards
What is the Gram staining procedure?
Smear culture on slide. Fix with ethanol. Apply crystal violet solution for 1 minute and wash. Apply Gram iodine solution for 1 minute and wash. Decolorize with alcohol/acetone for 15 seconds and wash. Counterstain with safranin for 1 minute and wash.
What colors do gram positive and gram negative bacteria end up with?
Gram positive retain the purple coloration due to the initial stains binding with the thick peptidoglycan layer. The gram negative bacteria appear red due to the decolorizing step washing away the violet stain and the safranin stain replacing it.
What is a selective medium?
A selective medium is a medium that permits the growth of organisms that one seeks while repressing the growth of other organisms. MacConkey agar contains crystal violet, which inhibits the growth of gram positive bacteria while allowing the growth of gram negative.
What is a differential medium?
A differential medium uses a particular bacterial biochemical property to allow for the visible differentiation of one organism from another. MacConkey agar differentiates organisms based on their ability to ferment lactose. Fermentation results in decreased pH, and the agar contains lactose and a pH indicator which results in red/pink colonies of lactate fermenting bacteria.
What is an enrichment medium?
An enrichment medium provides a selective advantage for one organism over others in a polymicrobic specimen. It may include a chemical that is relatively less toxic to one microbe (selenite is less toxic to salmonella) or conditions that are more favorable to one microbe (Yersinia grows well at 4 degrees).
What is blood agar and what is it useful for?
Blood agar is a differential, non-selective medium that supports most medically important species. It is very useful for visualizing hemolysis positive colonies.
What is psuedomonas isolation agar (PIA)?
PIA is a selective medium formulated to enhance the formation of pigments in Psuedomonas, and contains broad spectrum antimicrobials that do not affect psuedomonas.
